"So far, eight people are dead and we have sent dozens of others to hospital for treatment," said Bui Huu Loi, chairman of the Bat Xat district of Lao Cai province, where the accident occurred.
"We don't know whether (foreigners) were on board. We are working on it," he told AFP.
The state-run Thanh Nien online newspaper said that at least 49 people had been on the bus, which was travelling back from popular tourist hill station Sapa to Lao Cai, the province's main city.
According to the Lao Cai online newspaper, the valley was around 200 metres deep.
Tomorrow is Vietnam's National Day and a public holiday so there are more local tourists on the road than usual.
Road accidents are common in communist Vietnam and kill about 35 people every day. Narrow highways, poorly maintained vehicles and drivers' disregard for road safety and traffic rules are blamed for most fatalities.